Flue-gas desulfurization facilities of the wet type in which sulfur dioxide is removed from flue gases by means of an absorbent slurry have recently come into wide-spread use. In those systems the absorbent slurry is injected into the path through which the flue gas passes so as to ensure efficient contact between the slurry and the gas. The injection produces a mist of the absorbent slurry, which must eventually be eliminated.
FIG. 2 illustrates a typical arrangement (of essential components only) of the conventional wet flue-gas desulfurization systems. It comprises an absorption column 1, a tank 2 formed at the bottom of the absorption column 1 and which is supplied with an absorbent slurry S, for example, of limestone through a slurry supply line not shown, a circulating pump 3 for forcing the slurry upwardly from the tank 2 into a flue gas inlet 1a at the top of the absorption column where the slurry comes in contact with the flue gas, an agitating bar 7 supported by a rotating shaft 5 hanging from the ceiling 4 of the tank 2 and which is driven by a motor 6 to turn horizontally in the bath of slurry S, a gas-outlet duct 8 formed to rise integrally from the top of an end part of the tank 2 and extend sideways, and a mist eliminator 9 installed like a partition across the duct body 8a of the gas-outlet duct 8.
In the system, flue gas A to be treated is introduced into the absorption column 1 at the flue gas inlet 1a, where it is contacted with the absorbent slurry S being sprayed from a header pipe 10. In this manner, the crude flue gas A is freed from sulfur dioxide by absorption and is discharged as a clean treated gas B from the gas-outlet duct 8. The absorbent slurry S that has been sprayed out of the header pipe 10 flows down, while absorbing sulfur dioxide from the flue gas, by way of a bed of packing material 11. The absorbent slurry S that has fallen into the tank 2 is stirred by the agitating bar 7 and is oxidized through contact with numerous bubbles of air produced by air supply means not shown, and then is taken out as gypsum (a by-product).
Meanwhile, inside the gas-outlet duct 8, the mist in the treated gas B is captured by the mist eliminator 9, caused to flow into a drain hopper 8b formed at the bottom 8c of the duct body 8a, and is returned to the tank 2. The mist eliminator 9 is usually equipped with a nozzle (not shown) which sprays wash water against an element of the eliminator, and the washings too are allowed to flow down through the drain hopper 8b into the tank 2 for reuse as a part of the absorbent slurry S.
FIG. 3 shows another arrangement (of essential components) of a prior art wet flue-gas desulfurization system disclosed in Japanese Utility Model Provisional Publication No. 62-130719. Parts like those shown in FIG. 2 are designated by like numerals. In this system the mist eliminator 15 is suspended from the ceiling of the tank 2 toward the liquid surface of the absorbent slurry S and is joined at the lower end to a partition plate 16 which in turn extends partly into the bath of the slurry. The treated gas is freed from mist as it passes between the liquid surface of the absorbent slurry S and the ceiling 4, and the captured mist and washings are directly flown down into the bath inside the tank 2.
The wet flue-gas desulfurization system of the construction shown in FIG. 2 requires the drain hopper 8b and piping for connecting the hopper to the tank 2. This presents a shortcoming of additional cost for the complex construction. Another disadvantage is the high maintenance cost with frequent cleaning for the removal of said deposits (gypsum and the like in this case) from the bottom 8c of the gas-outlet duct. The duct body 8a of the gas-outlet duct 8 cannot have a fairly large diameter for cost reason. Accordingly, the cross sectional area of the gas flow path is limited and the flow velocity increased, for example, to about 18 meters per second. Thus a large mist removal capacity cannot be secured, and the mist concentration on the outlet side of the mist eliminator 9 (the final mist concentration in the treated flue gas) can hardly be lowered.
The wet flue-gas desulfurization system of FIG. 3 is simpler in construction without the need for the drain hopper and associated parts. However, the difficulty in lowering the outlet mist concentration is more serious. Increasing the height of the ceiling 4 of the tank 2 from the bath level would entail various drawbacks; for instance, it would call for a taller absorption column 1 with more component parts at extra cost and a greater lift of the circulating pump 3 would boost the power consumption. For these reasons the ceiling 4 has to be kept low, with the consequences that the cross sectional area of the flow path between the ceiling 4 and the bath surface decreases and the velocity of gas flow through the path increases, for example, to from 6 to 10 m/s, thereby reducing the mist-collecting capacity of the mist eliminator 15 accordingly. Furthermore, because the mist eliminator 15 is installed close to the region where the absorbent slurry S issued from the header pipe 10 falls down, the mist load at the inlet of the mist eliminator 15 is very heavy (i.e., the mist concentration immediately upstream of the mist eliminator 15 is very high). This, as a result, poses still another problem of a substantially high outlet mist concentration.